At the core, it is probably unsound (both Jakob and Wind think so), but the discovery that even something like this is playable at a fairly high level enriches chess.

White's idea is that after Black takes the knight, White can roll the central pawns forward and bring Black's knights to passive positions. One of the most popular lines gives White a dangerous attacking position with a strong pawn on d6: 4...Nxe5 5.d4 Nc6 6.d5 Ne5 7.f4 Ng6 8.e5 Ng8 9.d6 cxd6 10.exd6.

Unfortunately for White, this isn't the end of the story because Black can take over the attack by sacrificing the rook on a8 10...Qf6 intending 11.Nb5 Nxf4 12.Nxc7+ Kd8 13.Nxa8 Qe5+ (though White could try 11.Qe2+ Kd8 12.Ne4 instead, which might give enough compensation). However, in practice Black rarely finds that continuation, and if Black grovels to the defence of c7 by playing ...Kd8 then White generally gets sufficient compensation for the piece. Note that if Black plays the other knight retreat at move 6, 6...Nb8, then 7.e5 probably gives White at least sufficient compensation, as White will establish a pawn on d6 and Black is left with no counterplay.

More common is 5...Ng6 6.e5 Ng8 7.Bc4. Then Black has a choice between 7...Bb4, 7...d6 and 7...d5. 7...Bb4 appears to give White reasonable compensation for the piece after 8.Qf3 Qe7 9.h4, but 7...d6 intending 8.Qf3 Qd7 is more critical, after which White probably falls a little short of full compensation, but retains attacking chances. 7...d5, recommended by Max Euwe, is more of a test, returning one pawn in order to free the black position. After 8.Bxd5 c6 (Euwe) or 8...N8e7, White retains some, but not quite enough, compensation for the piece thanks to the strong array of central pawns.

The Halloween Gambit is thus the sort of line that can provide fans of unorthodox openings with a lot of fun in rapid and thematic games, and just analysing the unusual positions that arise. It is easy to underestimate and just one or two inaccurate moves from Black can leave White with good compensation for the sacrificed knight, leading to attractive attacking wins.

However, I cannot recommend it as a serious tournament weapon because in addition to the issues mentioned above, Black has various ways to return the piece and reach at least an equal game, and at least one way to get a slight advantage by doing so. 5...Bd6 6.dxe5 Bxe5 is pretty good, while 5...Nc6 6.d5 Bb4, recommended by Jan Pinski, gives Black the majority of the attacking chances, e.g. 7.dxc6 Nxe4 8.Qd4 Qe7 9.Qxg7 Nxc3+ 10.Be3 Nd5+, or 9.Be3 f5 10.cxd7+ Bxd7. I have seen the Halloween compared with the Cochrane Gambit against the Petroff Defence, which is also probably better for Black with best play, but in the Cochrane Black has no good way to return the piece and reach equality, let alone a better position, in the opening, and must instead seek salvation in the accepted lines of the gambit and let White have the majority of the attacking chances, and that is why I regard the Cochrane to be more viable than the Halloween for serious over-the-board play.

Games and analysis

The illustrative examples feature the following lines:1. 4...Nxe5 5.d4 Nc62. 4...Nxe5 5.d4 Ng6 6.e5 Ng8 7.Bc4 d53. 4...Nxe5 5.d4 Ng6 6.e5 Ng8 7.Bc4 Bb44. 4...Nxe5 5.d4 Ng6 6.e5 Ng8 7.Bc4 d6In the notes I also mention some of Black's ways to return the piece and reach an equal or better game, but the games also illustrate White's attacking potential in the lines where Black holds onto the extra piece but goes wrong.